Nowadays a mobile phone, digital camera, notebook, netbook, tablet etc. has become daily used electronic device. The power of the batteries of these electronic devices decides the length of time of use of the electronic device. Electronic devices such as a mobile phone, digital camera, notebook, netbook, tablet etc. must be used with corresponding battery chargers. Generally, these battery chargers (or called as power supplies) are unique to the corresponding electronic devices.
Typically, each battery charger is provided with an input terminal, such as a USB terminal, for the connection to the power line for charging the battery and simultaneously powering the device connected to the battery. FIG. 1 shows a battery charger 1 having an input terminal Vline connectable to a supply voltage and an output terminal connectable to a battery unit 2; the supply circuit 3 of the battery charger 1 which provides the current Ibat to the battery unit 2 may be controlled by one among the different regulation loops: input voltage loop IVL, input current loop ICL, thermal loop TL, current battery loop CBL, battery voltage loop BVL.
Usually, the battery charger is used to manage energy flowing from the input source Vline into the battery unit 2, regulating the charging current according also to the voltage level of the battery. In particular, when the battery voltage reaches the programmed floating voltage level (VFLT) the charge is stopped and the current Ibat flowing from the battery charger to the battery unit 2 is interrupted as consequence. Floating voltage accuracy is crucial for the battery lifetime and for safety reason as well. Some chargers also implement other control loops in the same chip able to limit the input current or to regulate the charging current in reason of the input voltage level or the silicon temperature. Generally, the PWM control signal is results of the combination of those loops.
A typical charging profile is shown in the FIG. 2. Two main charging phases are clearly identified; a first charging phase wherein the current Ibat is kept constant while the voltage Vbat rises, and a second charging phase wherein the voltage Vbat is kept constant and the current Ibat goes down to the level Iterm.
Until the battery voltage is below the VFLT voltage level, the current Ibat flowing into the battery is kept constant by a current loop regulation. This phase is named constant current (CC).
The charging current regulation during CC phase can be the combination of different regulation loops (input voltage loop IVL, input current loop ICL, thermal loop TL, current battery loop CBL, etc) one at a time. So, the charging current level during CC change can vary as consequence, as shown in FIG. 2 wherein the three charging current levels are present Itrk during the trickle-charge TRK, Ipre during the pre-charge PRE after the trickle-charge and Ifast during the fast charge FAST after the pre-charge.
Once the battery voltage Vbat is close to the level VFLT, the current starts to decrease and the regulation is taken by the battery voltage loop BVL. This phase is named constant voltage (CV).
Usually, the CV phase comprises the phase TAPER which ends when current Ibaty into the battery reaches the current threshold Iterm. After that, the end of charge (EOC) is reached and the charger is switched in off state OFF.
There are known in the state of the art control circuits, so called “Fuel Gauge”, to provide an estimation of the charging level of the battery.
The use of the Fuel Gauge during charging can assure estimation of the battery charging level with high precision (˜0.1%). Typically, the accuracy is achieved by means of an analog to digital converter and a sigma delta modulator that provides the mean value of the voltage level with a rate of few milliseconds. The Fuel Gauge and the battery charger operate separately.
However, the introduction of a fuel gauge circuit causes an increase of the silicon area.